Newswise — Being resistant to aspirin makes patients four times more likely to suffer a heart attack, stroke or even die from a pre-existing heart condition, according to a study published on bmj.com today.

The study relates to patients who are prescribed aspirin long term as a way of preventing clots from forming in the blood.

Patients who are labelled "aspirin resistant" have blood cells (platelets) that are not affected in the same way as those of patients who are responsive to the drug, ie people who are "aspirin sensitive."

There is currently no agreed method of accurately determining who is and isn't aspirin resistant and the reasons why someone might be aspirin resistant are currently a cause of controversy.

Relatively few studies have looked at whether aspirin resistance has any impact on clinical outcome so the Canadian authors carried out a review of all the available data to better understand the relationship between the two.

They identified 20 studies, involving 2,930 patients with cardiovascular disease, all of whom had been prescribed aspirin as a way of preventing clots from forming in the blood. 28% were classified as aspirin resistant.

They found that all aspirin resistant patients, regardless of their underlying clinical condition, were at greater risk of suffering a heart attack, stroke or even dying. In particular they found that 39% of aspirin resistant patients compared to 16% of aspirin sensitive patients suffered some sort of cardiovascular event.

They also found that taking other drugs to thin the blood, such as Clopidogrel or Tirofiban, did not provide any benefit to these patients.

The authors conclude that there needs to be further studies on aspirin resistance to identify the most useful test to determine the condition. They also say aspirin resistance: "is a biological entity that should be considered when recommending aspirin as antiplatelet therapy."